Relational databases are used for storage and retrieval of information. The information is structured in the database as two-dimensional tables of rows and columns. A column heading designates the type of data stored in each column.
Users are able to access the database information typically by using database management software. The database storage media, management software, and other hardware and software components together make up a database management system, or DBMS. The database management software provides specialized commands for accessing and manipulating the database information. Such commands are according to a standard database-query language, such as a Structured Query Language (SQL).
Traditionally, a DBMS processes queries in batch mode. In other words, a user wanting to extract information from the database would submit a query, wait some amount of time during which no feedback is provided, and then receive an answer.
It is increasingly common for a DBMS to present progressively refined intermediate results to a query during processing of the query. The intermediate results are displayed typically along with a “confidence” factor. For accurate intermediate results, random sampling is used. However, in a parallel DBMS having multiple nodes, randomness may be lost if one node produces a result (in response to a query) faster than another node. This may lead to a skewing of results so that intermediate results are more likely to have a low confidence factor.